2.6 Hydrocarbons from natural gas - Treccani, il portale del … · · 2018-03-2816% of the total...

2.6.1 Introduction

Natural Gas (NG) reserves have constantly increasedover recent decades and currently exceed proved oilreserves. At the beginning of 2002, the world’s naturalgas reserves were estimated at 1,080 billion barrels ofoil equivalent (boe), whereas in the same period theproved oil reserves were 1,032 billion boe.Approximately 80% of gas reserves are located in 12countries and the sum of the amounts present inRussia and Iran reaches approximately 50% of thetotal; this sum rises to about 70% if the other MiddleEast states are included. 30% of the overall reserves isdivided among the other 10 countries, in Asia/Oceania(about 10%), Africa (about 7%), Europe (about 5%),South America (about 5%) and North America (about4%). In many of these countries, gas reserves are ofconsiderable strategic importance; it is sufficient to

consider, for example, the Caspian region, where thegas and oil reserve ratio varies from 1.4 (Kazakhstan)and 36 (Turkmenistan), or Qatar, where this ratio isapproximately 14. For these countries and otherareas, such as the continental platform of NWAustralia or West Africa, the economic growthprospects depend to a large extent on the possibilityof bringing these reserves to market outlets asconveniently as possible.

The natural gas market must currently beconsidered a regional market; of the 2,330 billion m3

produced in the world in 1999, only 20% has beencommercialized on a long-distance scale: 15% viamethane pipelines and 5% as Liquefied Natural Gas(LNG).

16% of the total proved natural gas reserves can bedefined as ‘remote’, as their geographical locationmakes transfer to the potential destination markets

161VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY

2.6

Hydrocarbons from natural gas

ArcticCoastal Plain1,136 Gm3

East Venezuela671.1 Gm3

Talara Basin(Peru)

51.0 Gm3

Nigeria1,670.7 Gm3

Zagros Mountains(Iraq)

611.6 Gm3

Zagros Mountains(Iran)

2,364.5 Gm3Arabian

Coastal Province(Kuwait)56.6 Gm3

Precaspian Basin(Kazakhstan)373.8 Gm3

ArabianCoastal Basin(Saudi Arabia)

410 Gm3

Fig. 1. Most attractive remote gas basins (Petroconsultant MAI-ZEUS, 1999).

extremely difficult with the existing technologies(methane pipelines or LNG). Fig. 1 shows a mapestimating the most relevant remote gas reserves.

From a historical point of view, long-distancemethane pipelines represent the first technologyused for bringing gas reserves to the market, as inthe case of the Soviet Union and Algeria. Theinvestment costs, however, are still extremely high.Furthermore, crossing the borders between nationsoften creates, in most geographical areas, problemsrelating to geopolitical stability between producerand consumer countries. New projects for long-distance methane pipelines are, in fact, at presentonly conceived for those areas in which there are noalternatives for gas exploitation. As an example, thetrans-Caspian methane pipeline project can be cited,which should transport 30 billion m3 of gas fromTurkmenistan to Europe, through Azerbaizhan,Georgia and Turkey, with an investment estimated atabout 2.5-2.7 billion dollars and with geopoliticalproblems yet to be solved, or the pioneering BlueStream project for the deep-water laying of amethane pipeline, recently carried out by Eni in theBlack Sea.

Liquefied Natural Gas (LNG) remains aninteresting alternative for the exploitation of remotegas; improved technologies and consolidatedimplemented experience have lowered the cost of thistechnology allowing the fulfillment of what untilrecently was considered impossible. The LNG market,therefore, seems destined for extensive development inrelation to the world’s economy prospects.

According to a Cambridge Energy ResearchAssociates study (CERA, 2002), the current LNGcapacity, which is 119 million t/y as a result ofdeclared projects, should increase by 34 million t/ythrough expansion projects and by 111 million t/ythrough new projects, to reach 264 million t/y in 2020.This market volume is already very close to theinstalled capacity of re-gasification terminals,estimated at between 260 and 310 million t/y for 2020.The construction of new LNG terminals is hindered bythe identification, in the destination markets, ofgeographical areas suitable for the construction ofthese plants. Thus, a further expansion of theinternational commercialization of natural gas islimited by the necessity of constructing costly andcomplex infrastructures (LNG terminals and plantsand long-distance and high-pressure methanepipelines) and establishing long-term contracts inareas affected by high geopolitical turbulence.

These premises reflect the importance oftechnologies for the conversion of natural gas intoliquid products. Among all the possible technologicalalternatives for said conversion (Fischer-Tropsch

conversion, synthesis of methanol and synthesis ofdimethyl ether), only the former does not requirespecific logistic structures and the opening of newmarkets. It is probably for these reasons that the majoroil companies have shown great interest, since thebeginning of the 90s, in Fischer-Tropsch synthesis,which will be examined in more detail below.

Historical outline1923 witnessed the discovery of the hydrogenation

reaction of carbon monoxide for the synthesis ofhydrocarbons; this reaction was subsequently calledFischer-Tropsch synthesis after the name of itsinventors, Franz Fischer and Hans Tropsch,researchers at the Kaiser Wilhelm Institut fürKohlenforschung in Mühlheim-Ruhr, in Germany.

Fischer and Tropsch perfected the conversion ofsynthesis gas, a gaseous mixture consisting of carbonmonoxide and hydrogen, into liquid hydrocarbons,operating at atmospheric pressure and using catalystsbased on precipitated cobalt. The Fischer-Tropschprocess was subsequently developed on an industrialscale by various German companies that adoptedmolten iron as a catalyst. During the Second WorldWar, in fact, the main objective was the production ofsynthetic fuels (synfuel), mainly diesel, for use inengines. The energy problem that Germany was facingat that time, due to the lack of crude oil reserves,created favourable conditions for developing theFischer-Tropsch process, mainly due to the availabilityof large coal reserves, which could be used as rawmaterial for the production of synthesis gas. It can beestimated that the maximum production, reached in1944, was around 16,000 barrels/day, for a total of4,500,000 barrels of synfuel produced throughout thewhole period of the war.

In addition to Germany, the United States and theUnited Kingdom were also active in developing theFischer-Tropsch process, even if the circumstances inthese countries were not such as to make synthetic fuelcompetitive with respect to traditional fuels. At the endof the Second World War some German scientists whowere involved in studying the Fischer-Tropsch process(for example H. Pichler, a member of F. Fischer’steam) moved to the United States to continue theiractivity at the Bureau of Mines. In 1944, in fact, thisoffice was commissioned by the American governmentto develop a study on synthetic fuels (Synthetic FuelsAct); for this purpose, the Bureau of Mines requestedthe transportation of two German Fischer-Tropschplants to Louisiana and Missouri, considering them aswar indemnity.

At the end of the 1940s, Hydrocarbon Researchdeveloped, in Bronsville (Texas), a new Fischer-Tropsch process called Hydrocol, which was based on

162 ENCYCLOPAEDIA OF HYDROCARBONS

HYDROCARBONS FROM NON-CONVENTIONAL AND ALTERNATIVE FOSSIL RESOURCES

the German technology. The unit entered intoproduction in 1950; the feed consisted of natural gasand production was approximately 8,000 barrels a dayof fuels. Apart from start-up problems, for which theplant had to be redesigned, the enterprise proved to beuneconomic as a result of the high production costs.The Hydrocol plant was subsequently entirelypurchased by one of the partners, Texaco, which, in themiddle of the 1940s, had initiated its research anddevelopment activity in the gasification field, togetherwith initiatives on the Fischer-Tropsch conversion. In1947, Texaco built its first Fischer-Tropsch plant of120 barrels/day, at Montebello (California). Theunfavourable economic situation i.e. the drop in theprice of crude oil and increase in the cost of naturalgas, led to the abandonment of this unit, with the onlyexception of the synthesis gas generation section, usedfor implementing the Texaco gasification technology.

At the end of the 1940s, Sasol, a South Africanindustry, also began studying the Fischer-Tropschprocess. The first plant, Sasol I, derived fromHydrocol, started up in 1955. Sasol II and Sasol IIIcame into production in the 1980s; the three units werefed with coal and aimed to produce gasoline. It shouldbe noted that the limited availability of crude oil andthe political climate of the time, which culminated inthe total embargo of South Africa, favoured theexploitation of huge coal reserves on the part of theSouth African government, and consequently thedevelopment of the Fischer-Tropsch process. The threeplants are currently still in production together with anatural gas conversion plant (Mossgas). The overallproduction of Fischer-Tropsch products can beestimated at approximately 160,000 barrels/day forSasol I, II and III; the Mossgas plant, thanks in part tosubsidies from the South African government,currently produces 23,000 barrels/day.

The 1950s marked the first construction of aFischer-Tropsch plant with technology based on a gas-liquid-solid reactor with a slurry catalyst, rather thanthe tubular fixed-bed reactor adopted until then. Thetechnology, applied by Rheinpreussen-Koppers ofHomberg-Niederhein in Germany, was developed byH. Kolbel and P. Ackermann. The feed consisted ofsynthesis gas produced from coal, whereas thecatalytic system was based on iron.

In the 1970s, new initiatives were conceived withrespect to the Fischer-Tropsch technology, due to a risein the price of oil.

Gulf Oil, together with Badger Engineering,started operating on cobalt-based catalyst systems andin 1979 a pilot unit of 35 barrels/day was activated andran for several years. Chevron then purchased Gulfand, following another drop in the price of crude oil, inthe late 1980s, it decided to hand over the R&D

Fischer-Tropsch Gulf/Budger division to Shell (RoyalDutch/Shell Group).

Exxon began studying the Fischer-Tropsch processin their New Jersey research centre, in 1973, the sameyear in which the embargo on petroleum was exertedby the Arab countries. The name of the technologydeveloped by Exxon is AGC-21 (Advanced GasConversion for the 21st century). The cost of theresearch and development phase was considerable:approximately 300 million dollars. The AGC-21 unitwas created in 1989 in the Baton Rouge refinery, inLouisiana, and has a capacity of 200 barrels/day. TheAGC-21 technology for the conversion of natural gasinto liquid hydrocarbons was tested for a period ofthree years i.e. until 1992.

In 1980, Mobil began studies on the Fischer-Tropsch conversion constructing a small pilot unitbased on iron catalysts. In 1983, Mobil decided tointerrupt research on the Fischer-Tropsch process,considering it uneconomic; an industrial unit,however, for the conversion of natural gas to methanoland the subsequent transformation of Methanol toGasoline (MTG) of 14,500 barrels/day was set up atMontuni, in New Zealand. This alternative project tothe Fischer-Tropsch conversion was a success from atechnical point of view but not economically, so muchso that the plant is currently used only for theproduction of methanol. Shell started developing itstechnology at their Amsterdam research centre in1973. The research and development activity whichled to the development of the Shell Middle DistillateSynthesis (SMDS) technology was concluded in1990, with an investment of approximately 1-1.5billion dollars. In 1989, Shell announced theconstruction of a 12,500 barrels/day plant in Bintulu,Malaysia, for the production of Fischer-Tropschproducts, at a capital cost of 850 million dollars. Theplant, which is capable of processing 2.8 million m3

of natural gas per day using cobalt catalysts, went intoproduction in 1993. The installation, which wasseriously damaged by the explosion of the cryogenicair fractionation unit in December 1997, wasreactivated in 2000.

2.6.2 Gas to Liquids process bymeans of the Fischer-Tropschreaction

In its most modern version, the Fischer-Tropschsynthesis is applied to the upgrading of natural gas bymeans of a three-step process scheme which allowsliquid fuels to be obtained starting from methane:• Production of synthesis gas i.e. of the mixture

consisting of carbon monoxide and hydrogen,


HYDROCARBONS FROM NATURAL GAS

obtained by the reaction of natural gas with oxygenand/or steam.

• Production of hydrocarbons, by means of Fischer-Tropsch synthesis, wherein synthesis gas isconverted, through a polymerization mechanism,into saturated linear hydrocarbons prevalentlyconsisting of wax (syncrude).

• Transformation of the wax into liquid products(diesel fuels, fuels for aircraft and chemicalproducts) by means of hydrocracking and hydro-isomerization reactions. The core of the technology lies in the second step,

the Fischer-Tropsch synthesis. In the technologyadopted in Germany in the first half of the Twentiethcentury, the raw material used for the production ofsynthesis gas was coal. The formation of methane orlight paraffins in the Fischer-Tropsch section wasaccepted, as these hydrocarbons could have beenusefully adopted in the production system. Nowadays,with natural gas as a raw material, the production oflight hydrocarbons in the Fischer-Tropsch steprepresents a net economic loss and must therefore beavoided. Modern technology advancement has beenmainly oriented towards solving this problem and themost significant progress has been the development ofnew supported cobalt-based catalysts and the use ofgas-liquid-solid reactors with a slurry catalyst (SBCR,Slurry Bubble Column Reactor). These solutionsprevent the formation of light hydrocarbons but themain reaction product consists of paraffinic waxwhich, in a second step (hydrocracking process) mustbe converted into the desired products.

Synthesis gas production

Synthesis gas can be produced from various fossilsources, such as natural gas, naphtha, residual oils,coke from petroleum and coal. Natural gas, however,is the raw material of greatest interest, above all ifavailable at a low cost. The generation step ofsynthesis gas is based on technologies widely-testedon an industrial scale; their application to the Fischer-Tropsch process, however, requires considerable workfor its integration, optimization and processmodifications in order to obtain the right H2/CO ratio,the maximum efficiency and minimize investments.For this reason, various companies involved in thedevelopment of a Fischer-Tropsch technology, haveadopted different solutions for the production ofsynthesis gas. The selection of the productiontechnology of synthesis gas is of fundamentalimportance in the economy of the entire conversionprocess of the gas into liquid hydrocarbons; economicestimates, in fact, refer to investment costs, for theproduction section of synthesis gas, as higher than half

of the total cost for the construction of the wholeplant. In the production cost of a barrel of Fischer-Tropsch product for a plant of 100,000 barrels/day, theproduction unit of synthesis gas accounts for a total of33% of the product cost and 60% for the cost of theplant (ADL, 1998).

The methods used for the production of synthesisgas from natural gas can be divided into two groupsdefined by the main chemical reaction which leads tothe formation of the mixture of CO and H2.

The partial oxidation of natural gas with air,enriched air or oxygen is characterized by thefollowing reactions: • main reaction

CH4�1/2O2��CO�2H2 DH°��36 kJ/mol• secondary reactions

CO +1/2O2��CO2 DH°��284 kJ/molH2 +1/2O2��H2O DH°��242 kJ/molThese reactions are extremely exothermic, and thus

take place with the production of a high quantity ofheat. As the reaction environment is oxidizing, theformation of a carbonaceous residue does notrepresent a problem; the over-oxidation of the reactionproducts, however, which leads to the formation ofcarbon dioxide and water, must be controlled so as notto jeopardize the yield of synthesis gas and to preventan excessive production of heat. The partial oxidationreaction produces a synthesis gas containing about twohydrogen moles per mole of carbon monoxide. Thiscomposition is the best, when the synthesis gas is usedto feed a reactor for the production of heavyhydrocarbons, by means of the Fischer-Tropschreaction, with a cobalt-based catalyst.

In the absence of the catalyst, the reactiontemperature must be sufficiently high to reach the totalconversion of the methane. In partial oxidation, thetemperature of the outgoing gases is in the order of1,000-1,100°C. This stage is close to thermodynamicequilibrium composition, and the CH4/O2 ratiorequired in the feed should therefore be equal to 2; assecondary reactions cannot be excluded, however, thisratio is richer in oxygen (CH4/O2�1.4).

The direct catalytic oxidation of methane tosynthesis gas is a process in the development phaseand is more efficient compared to the previousprocess, but, at the same time, it is a more criticalprocess to handle. The presence of the catalyst makesit more difficult to control the reaction, which, as aresult of the presence of both methane and oxygen,could become flammable or explosive. The necessityof limiting the local concentration of oxygen and thepresence of a solid can lead to the undesired formationof carbonaceous deposits on the surface of the catalyst.

The catalytic systems used are based on nickel,mixed oxides of ruthenium and rare earth (Ln2Ru2O7),



or transition elements supported on alumina, or oxideshaving a perovskite-type structure containing nickel(e.g. Ca0.8Sr0.2Ti0.8Ni0.2O3), or other metals of groupVIII of the periodic system supported on silica and/oralumina.

The reaction of natural gas with steam can bedescribed as follows:• Main reaction

CH4�H2O��CO�3H2 DH°��206 kJ/mol• Water-gas shift reaction

CO�H2O��CO2�H2 DH°��41 kJ/mol • Coal formation reactions

2CO��C�CO2 DH°=�172 kJ/mol(Bouduard reaction)CO�H2��C�H2O DH°��133 kJ/molCH4��C�2H2 DH°��75 kJ/molUnlike the previous reaction, the main reaction

takes place with strong heat absorption. In addition tomethane, the carbon monoxide can also react withsteam, forming CO2 and further hydrogen. Thisreaction, which can be carried out separately in aspecific reactor, is generally used to regulate theH2/CO ratio and it is always adopted in processes forthe production of hydrogen. One of the criticalaspects of gas-steam reaction processes is theformation of carbonaceous residues which can occuraccording to one of the reactions indicated above; thethree reactions are listed in order of criticality. One ofthe methods for limiting the formation of acarbonaceous residue is the use of a highsteam/methane ratio in the feed to the reactor. As anincrease in this ratio also involves a cost increase, it ispreferable to use the lowest steam/methane ratiocompatible with the necessity of controlling theformation of carbonaceous residue. Typical values ofthis ratio range from 2 to 5. Various technologies areavailable which allow values of less than 1 to bereached and these are of particular interest for theproduction of synthesis gas, destined for Fischer-Tropsch conversion processes.

The methane conversions, in steam reactions, aretypically in the order of 90-92%, with a synthesis gascomposition at the outlet of the reactor similar to thatenvisaged by thermodynamic equilibrium. The catalystused is based on supported nickel, the temperature isabout 930°C and the pressure 15-30 bar.

The processes adopted industrially for theproduction of synthesis gas destined for Fischer-Tropsch conversion use both partial oxidation andsteam reaction; the most frequently used solution is acombination of both approaches in technologiesdefined as combined reforming processes. Thereactor-design technologies and combination ofreactions are optimized to minimize investment costs,optimize energy cycles and maximize the yield of

synthesis gas, at the same time optimizing the H2/COratio in the end-product. One of the most moderntechnologies developed consists of so-called AutoThermal Reforming (ATR) developed by the Danishcompany Haldor Topsøe, at the end of the 1950s. Theprocess combines partial oxidation and steam reactionin a single reactor. Natural gas, oxygen or enriched airand steam are fed through the reactor inlet. In a firstphase of the reactor, partial non-catalytic oxidationtakes place, which, in addition to partially convertingthe gas, produces the heat necessary for the steamreaction. The hot gases produced in the first step thenreach a catalyst, where the conversion is completed byreaction of the gas with water vapour.

Fischer-Tropsch synthesis

Thermodynamics The Fischer-Tropsch reaction is extremely

exothermic and mainly consists of a polymerizationreaction which produces long linear hydrocarbonchains according to the scheme:

nCO�2nH2��[CH2]n�nH2O DH��167,4kJ/mol CO

The �CH2� group, an intermediate productgenerated by the hydrogenation of CO, represents thebase unit responsible for the chain propagation. Thenature and combination of Fischer-Tropsch reactionsrepresent a complicated but flexible system allowingdiversified products to be obtained. The productquality varies significantly in relation to the reactionconditions and type of catalyst used. The production ofparaffins and mono-olefins can be represented by thefollowing reactions:

(2n�1)H2�nCO��CnH2n�2�nH2O (A)(n�1)H2�2nCO��CnH2n�2�nCO2 (B)2nH2�nCO��CnH2n�nH2O (A)nH2�2nCO��CnH2n�CO2 (B)Similar reactions can be indicated for the

production of other hydrocarbons such as, forexample, diolefinic or unsaturated cyclic compoundscontaining a triple bond.

The formation of oxygenated compounds(alcohols) is represented as follows:

2nH2�nCO��CnH2n�1OH�(n�1)H2O(n�1)H2�(2n�1)CO��CnH2n�1OH�(n�1)CO2

The reactions considered are subdivided into groupA and group B reactions on the basis of the formationnot only of hydrocarbons but also of H2O or CO2

respectively. The values relating to group B reactionsare obtained by adding the reaction enthalpy of thecorresponding group A to that of the water-gas shiftreaction.

The data relating to the reaction enthalpies areextremely important as one of the major critical



aspects of the Fischer-Tropsch process is heat removal.Excessive temperatures normally cause low yields ofheavy products, the formation of carbonaceousresidues and deactivation of the catalyst.

Competitive reactions, which can complicate thesynthesis, are coke deposition (H2�CO��C�H2O)and the Bouduard reaction (2CO��C�CO2). Both ofthese can cause deactivation of the catalyst leading tothe formation of the carbide species of the activemetal: xM�C��MxC.

The standard free energy values DG° of a reactionare correlated to the equilibrium constant Keq bymeans of the expression DG° ��RT lnKeq wherein Ris the gas constant and T the absolute temperature.

If the DG° is known for a typical Fischer Tropschreaction, this allows the equilibrium constant to beobtained, which in the case of paraffinic products isdefined as follows:

Keq�( pCnH2n�2pn

H2O)/( pH22n�1pn

CO)

wherein p represents the partial pressure of thesubstance considered.

Keq can be expressed in terms of molar fractions Nand total pressure P:

P2n Keq�( NCnH2n�2Nn

H2O)/( NH22n�1Nn

CO)

Fischer-Tropsch reactions evolve with a reductionin the number of moles and consequently theconversion at equilibrium, for a given temperature,rapidly increases with a rise in the pressure (Anderson,1956). In reality, the higher temperature and pressurelimits (400°C and 30-40 bar) are determined bychanges in terms of selectivity and the deactivationrate of the catalyst.

Polymerization kineticsNot all reactions which are thermodynamically

possible necessarily take place, as there may be anextremely low reaction rate for some of these, whichdoes not allow equilibrium conditions to be reached.In addition to thermodynamics, reaction kinetics musttherefore also be taken into account.

A catalyst is a substance capable of increasing thereaction rate to allow equilibrium conditions to bereached more rapidly. When a series of reactions isthermodynamically possible, the type of catalyst andoperating conditions determine the trend ofcompetitive reactions and consequently the selectivitytowards the various products. The selectivityspecifically expresses the tendency of the catalysttowards producing a certain distribution of products ina system in which other products arethermodynamically possible.

Due to the diversity of the products obtained withFischer-Tropsch synthesis, it is not surprising that

numerous models are proposed in literature to describethe reaction mechanism.

Each scheme provides a satisfactory representationof the spectrum of products deriving from the Fischer-Tropsch synthesis; the experimental demonstrationsused in support of the theory, however, are generallyindirect and can be interpreted in different ways. Thevarious models can be attributed to three mainschemes.

The first scheme assumes as first reaction step, thedissociative chemiadsorption of the CO molecule. Thecarbon atom is hydrogenated to �(CH2)x� specieswhich, according to a mechanism analogous topolymerization, give rise to a propagation process ofmethylene (�CH2�) units thus generatinghydrocarbon chains.

The second category of proposed mechanismsassumes, as first reaction step, a non-dissociativeadsorption of the CO molecule, which is directlyhydrogenated to oxygenated species. The chain growthtakes place by the elimination of H2O betweenC�OH and C�H vicinal species.

The third category of mechanisms is characterizedby the CO insertion reaction. Also in this case, the COmolecule is adsorbed non-dissociated on the catalystand is subsequently inserted between the M�H andM�C bonds, wherein M represents an active site ofthe catalyst.

It is generally agreed that, in the hydrocarbonchain propagation mechanism and CO insertion in theformation of oxygenated compounds, intermediates ofthe carbenic type (�CH2�) intervene.

Fig. 2 summarizes the general mechanism. In theinitial phase of the process, the CO is chemiadsorbed(a) on the active centre of the catalyst, becomingactivated (b). The activated complex can then



(a)CO

M

C

M

O

M

C

M

CH2CH3OH H2O

M

CH3

M

CH2 CH4

M

O HH

M

C

M

O

M

(b) (c)

(d) H2

H2

H2 H2

H2

H2

H

H2 H2

Fig. 2. Initiation and formation of C1 compounds.

dissociate (c) in separate units C…M and O…M. Thecarbon atoms can subsequently be hydrogenated to�CH2� or migrate forming carbonaceousaggregates. As an alternative, the activated complexM…C…O…M can be hydrogenated (d) forming anactivated complex CH2O. This species by hydrogenationcan lead to CH3OH or �CH2� and H2O.

The �CH2� species can thus be produced bythe insertion of hydrogen via (c) or (d). The (c)mechanism seems the most probable for catalystscontaining iron or cobalt.

The direct hydrogenation of �CH2� speciesleads to the formation of CH4, a phenomenon which ismore marked with catalysts based on Co and Nicompared to catalysts based on Fe.

The last step, i.e. chain termination, whichdetermines the type of products, can be effectedaccording to various methods which includedeadsorption and hydrogenation phases. The chaintermination leads to the formation of olefins andvarious oxygenated species (Fig. 3).

According to the scheme proposed by E. Iglesia(Fig. 4), on catalysts based on Co and Ru, the chaingrowth mechanism takes place through the addition ofmethylene units to the alkyl groups adsorbed on thesurface of the catalyst (Iglesia, 1997). These cande-adsorb by b-hydrogen extraction, forming lineara-olefins, or by the addition of hydrogen, forminglinear n-paraffins. b-hydrogen extraction is atermination process which is reversible under typicalFischer-Tropsch synthesis conditions. The a-olefinscan, in fact, be adsorbed on the catalyst and form otheralkyl species bound to the active site, thus increasingthe probability of obtaining long hydrocarbon chains.

Iron-based catalysts, normally operating attemperatures higher than those used with cobalt-basedcatalysts, have higher selectivity towards the formationof oxygenated compounds, internal olefins and

branched hydrocarbons. This suggests a chainpropagation mechanism by CO insertion. Morespecifically, Iglesia proposed that the insertion of CObetween the metal centre and the alkyl group adsorbedcauses chain termination, with the consequentproduction of alcohols, whereas b-hydrogen extractionis responsible for the formation of internal olefins.

Fischer-Tropsch kinetics can be describedaccording to the Langmuir-Hinshelwood model whichidentifies the total reaction rate with that of the slowstep, considering all the other reaction steps in virtualequilibrium.

The predominant opinion considers, as slow step,the reaction of the non-dissociated H2 molecule withthe CO molecule chemi-adsorbed on the surface of themetal, creating methylene units coordinated with themetal centre: H2�CO…M��M…CH2+H2O (Madonet al., 1993).

The reaction rate must therefore be proportional tothe partial H2 pressure and to the fraction of thecatalyst surface covered by CO: r�KpH2

QCO, the CObeing in competition, with respect to the adsorption,with CO2, H2 and H2O. It has been observed that theinfluence of CO2 on the reaction rate is negligible andconsequently the resulting kinetics equation is thefollowing: r�KpCO pH2

/(pCO�apH2O).The presence in this equation of the term pH2

onlyas a component of the numerator explains why thepartial hydrogen pressure strongly influences thereaction rate making it the predominant factor, at lowconversions.

The term relating to H2O adsorption depends onthe type of catalyst and for iron-based catalysts it isnot negligible (in this situation water has an inhibitingeffect on the reaction kinetics).

In kinetic models relating to cobalt-based catalysts,proposed in literature (Yates and Satterfield, 1991),contrary to the case of iron, the term relating to water



2CH2

M

CH � 2H2 H2O

M

R

C

M

O

C � �

M

O

M

CH

R

CH

M

CH2

R

M

CH �

M

CH3

CH

M

R

CH2

CH2�CH2

M

CH2

Fig. 3. Chain termination with the formation of olefins andoxygenated species.

Fig. 4. Hydrocarbon chain growth mechanism and possibleadsorption of the olefin.

(Cn�1) (Cn�1)* *(Cn*)

(Cn) olefins (Cn)paraffins

(Cn�1OH)alcohols

secondaryreactions

(Cm, Cn�m)paraffins

(Cn) paraffins



does not appear, confirming the absence of a negativeeffect thereof on the reaction rate (Table 1). In all theequations the inhibiting effect of CO is evident.

The distribution of the products obtained from theFischer-Tropsch reaction can therefore be described asa chain growth mechanism of the polymerization type(Dry, 1996). The model commonly used wasdeveloped by Anderson, Schultz and Flory and wasprocessed on a statistical basis in relation to theprobability of chain growth. Its mathematicalrepresentation is the following: Wn �n�(1�a)2an�1

wherein n is the number of C atoms in the product, Wn

is the weight fraction of the product and a is thegrowth factor, which can have a value ranging from 0to 1. The growth factor a can be described with thefollowing expression: a�rp �(rp�rt) wherein rp

represents the chain propagation rate and rt thetermination rate.

The value of a can be obtained by linearizing the following logarithmic expression:ln(Wn�n)�ln(1�a)2�(n�1)lna. Said growth factor ais characteristic of the reaction conditions and catalyticsystem (metal, carrier and promoter).

The highest values of a are obtained with a decreasein the H2/CO ratio in the feed and in the temperature, anincrease in the pressure and in the catalytic systemscontaining cobalt and ruthenium instead of iron.

The chain growth mechanism, governed by theAnderson, Schultz and Flory distribution, implies stronglimitations on the selectivity of the various products. InFig. 5 it can be seen how the maximum selectivity for theC2-C4 fraction is 56%, for gasoline (C5-C9) 39%, jet fuel(C10-C14) 22%, whereas the limit selectivity value fordiesel fuel 23%. The product distribution can bemodified with respect to the Anderson, Schultz andFlory model, altering the reaction trend, for example by

mol

ar f

ract

ion

0

0.2

0.4

0.6

0.8

1.0

α0 0.2 0.4

methane

C2-C4

gasolineC5-C9

jet fuelC10-C14

dieselC15-C22

waxC22�

0.6 0.8 1.0

Fig. 5. Selectivities calculatedby means of the Anderson,Schultz and Florydistribution function.

Table 1. Kinetic models relating to cobalt-based catalysts

Kinetic Equations Authors

rH2�CO�a p2H2

pCO�1 Brotz

�rH2�CO�a p2H2

pCO / (1�b p2H2

pCO) Anderson

�rH2�CO�a p2H2

pCO�0,5 Yang

�rH2�CO�a pH2

0,55 pCO�0,33 Pannell

�rCO�a pH2pCO

�0,5/(1�b pCO0,5)3 Rautavuoma, Van der Baan

�rCO�a pH2

0,68 pCO�0,5 Wang

�rCO�a pH2

0,5 pCO0,5 /(1�b pCO

0,5�c pH2

0,5�d pCO)2 Sarup, Wojciechowski

�rCO�a pH2

0,5 pCO0,5 /(1�b pCO�c pH2

0,5)2 Sarup, Wojciechowski

�rCO�a pH2pCO / (1�b pCO�c pH2

0,5)2 Yates, Satterfield

interception of the reaction intermediates or introductionof olefins in the reagents.

A fundamental role in determining the kinetics ofthe Fischer-Tropsch reaction and consequently thequality of the product is that relating to the effects ofmass transport. The overall reaction rate can beinfluenced by all those physical factors whichinfluence the mass transfer rate, between the differentphases, of the reagents and products. As a result ofthis, in addition to a lower reaction rate, a change inthe selectivity of the various products can be verified.Fig. 6 indicates the situations in which phenomena dueto mass transfer are verified: passage of the reagentgas into the interstices of the catalytic bed(inter-particle diffusion), mass transfer of the reagentthrough the liquid film formed by the products anddiffusion of the reagent and products inside thecatalyst particle (intra-particle diffusion).

The main effect of diffusive phenomena consists increating intra- and inter-particle temperature andconcentration gradients.

The extent of the phenomenon associated withexternal diffusion mainly depends on the fluiddynamics and geometry of the system i.e. the spacevelocity of the reagent fluid and interphase surfacearea (form and dimensions of the catalyst particles).

The diffusion rate of the reagent fluid and back-diffusion rate of the products, both in the interphasearea and inside the catalyst particle, create the relativeconcentration and temperature gradients. The diffusion

coefficients of the main molecules involved arecorrelated by the following equation:DH2

�DCO�Dparaffins�Dolefins; H2 has a greaterdiffusing capacity.

Internal diffusion phenomena, which depend onthe chemical and morphological structure of thecatalyst (pore dimensions, density of the active sites)and on the molecular dimensions of the substancesinvolved, are the main cause of changes in terms ofselectivity to the various products, as they modify thediffusion capacity.

Iglesia defined a structural parameter (c) linked tothe re-adsorption phenomenon of olefins, which can beused for correlating the selectivity to C5� products withthe structure of the catalysts. The trend of the cparameter defined as c�R2eQCO �rpores (R�particleradius, e�porosity, QCO�density of the active sites,rpores�average pore radius) is indicated in Fig. 7. Thefunction shows how, with an increase in the active phasecharge or its dispersion (QCO) the selectivity towards C5�

products increases (Iglesia et al., 1993). An analogouscorrelation reveals a decrease in the selectivity towardsmethane, with an increase in QCO due to an increase inthe olefinic re-adsorption phenomenon.

The c parameter therefore allows an optimumselectivity value to be defined, on the basis of thegeometrical characteristics of the catalyst. Particlediameters of 200 mm are indicated by Iglesia as theexperimental limit over which diffusion phenomena



Fig. 6. Representation of phenomena due to mass transfer.

1 2

2

3

31

diffusionand reaction

interfacetransfer

catalystpellet

liquidproduct

H2�CO

convection

catalystpellet

.molecular size

.site density

.gas diffusivity

.hydrodynamics

.space velocity

.interfacial area

gas-liquidinterface

bedinterstices

Fig. 7. Influence of the c structural parameter on the C5+ selectivity.

simulationdiffusioninhibited

chain growth

simulationdiffusionenhanced

readsorption

experimental data

lightolefins

lightparaffins

increasing site densityincreasing particle size

dispersion/support effectspellet size variationseggshell thickness variations

C5�

sel

ectiv

ity

(%)

70

75

80

85

90

95

100

χ (10�16 m)10 100 1,000 10,000

limit the transport of CO, jeopardizing the chainpropagation and consequently the selectivity towardsthe C5� products.

Fischer-Tropsch reaction catalystsFrom a chemical point of view, the catalyst must be

capable of favouring the adsorption of CO on thesurface of the metal, having a hydrogenating activitywhich is not excessively high in order to obtain limitedCH4 contents and allow the insertion of methylenegroups. It is currently known that transition metals ofgroup VIII are particularly active in the Fischer-Tropsch synthesis. The catalysts commonly used arebased on Ru, Fe and Co.

Ruthenium favours hydrocarbon productions witha high polymerization degree (a values tendingtowards unity), but its high cost and limitedavailability restrict its wide-scale use. Iron is veryeconomical, has a high selectivity towards olefins but

is very active in the water-gas shift reaction; itproduces considerable quantities of oxygenatedproducts and is rapidly deactivated due to thedeposition of carbonaceous residues. The highselectivity towards olefins is due to thechemiadsorption of CO on the metal, which takesplace to a greater extent with respect to thechemiadsorption of hydrogen. This difference damagesthe hydrogenation rate in general and consequentlyalso the hydrogenation rate of the olefins.

Cobalt, on the other hand, which is known forbeing more hydrogenating than iron, has a lowerolefin/paraffin ratio. It is slowly deactivated andproduces limited quantities of oxygenated productsfavouring the formation of heavier products andhydrogenation reactions. Due to its high cost, it is usedin dispersed form on carriers such as alumina, silicaand titania (TiO2). Table 2 shows a comparisonbetween catalysts based on Co, Fe and Ru



0

wei

ght f

ract

ion

classical Co catalysts

wax diesel jet fuel gasoline LPG C1-C2

new Co catalysts

classical Fe catalysts

0.2

0.4

0.6

0.8

1.0

a0.75 0.79 0.83 0.87 0.91 0. 990.95

Fig. 8. Productdistribution in the Fischer-Tropschreaction.

Table 2. Activity of catalysts based on Co, Ru, Fe and without promoters

CatalystT�480 K, P�1 bar,

H2/CO�2NCO 103/s % CO2

produced% Olefins

C3-C7a

15% Co/Al2O3 17 1 54 0.90

non-supported Fe 1.4 31 94 0.44

11% Ru/Al2O3 1.8 – 88 0.69

3% Ru/Al2O3 1.5 4 65 0.70

(Bartholomew, 1991). The data are obtained at 480 K,1 bar, H2/CO equal to 2 and at a low CO conversion(1-10%), thus making diffusive limitation phenomenanegligible. The catalysts considered have a highconcentration of metal combined with a lowdispersion, properties necessary for limiting the effectof the carrier. In the first column, the values relating tothe number of catalytic events (turnover frequency,expressed as molecules of converted CO per catalystsite in the time unit) show how Co is much moreactive than Fe and Ru. The high activity of Fe withrespect to the water-gas shift reaction, which results inthe formation of a considerable quantity of CO2, canalso be observed.

The contribution of the water-gas shift reactiondepends on the conversion degree at which the catalystoperates and causes the change in productivity ofcatalysts based on Fe. The CO fraction converted tohydrocarbons decreases with an increase in the overallconversion of the CO (Davis, 1999).

The distribution of products obtained from theFischer-Tropsch reaction varies with the applicationfield relating to Fe and Co catalysts. In Fig. 8 it can beobserved how the new-generation cobalt-basedsystems give rise to a product which is richer in highmolecular weight hydrocarbon fractions.

Chemical promoters have a prominent role in theproduct distribution. Promoters have a multiple effecton the catalytic system, but they can be divided intovarious groups in relation to their function (Jager andEspinoza, 1995). For example, promoters such as K,Na, Mg, Sr, Cu, Mo, W and metals of group VIIIessentially increase the activity; in particular, lowcontents of alkaline metals (Na and K) increase thereaction rate and inhibit the formation of lighthydrocarbons and especially methane. From achemical point of view, by acting as electron donors,they weaken the M�H interaction and the C�Obond of the carbon monoxide adsorbed on the surfaceof the metal, reinforcing the M�C bond and causingan increase in the chain length. By favouring thebreakage of the C�Oads bond, which represents theslow reaction step (rate determining step), saidpromoters allow an increase in the reaction rate. Thenoble metals of group VIII (Ru, Re, Pt, Pd) also have apositive effect on the specific activity of cobaltsystems as they increase their reduction degree tometal. Ru, ZrO2, rare earth oxides (REO), Ti increasethe probability of propagation and therefore theselectivity towards high molecular weighthydrocarbons. Ru, REO, Re, Hf, Ce, U, Th, in the caseof Co, favour the possibility of regenerating thecatalyst. Other elements, such as Mn and Zn, inhibithydrogenation reactions favouring the production ofolefins.

The role of the carrier is fundamental for themechanical stability of the catalyst. In the case of gas-liquid-solid reactors with slurry catalyst, in particular,the catalyst must have good mechanical resistance toprevent fragmentation and abrasion phenomena, whichlead to the formation of fine particles with theconsequent loss of catalyst or contamination of theproduct. Studies on the effect of the carrier for cobaltcatalysts have demonstrated a greater resistance tofriction with an increase in the quantity of metalpresent for catalysts supported on Al2O3 compared tothose on SiO2 and TiO2 (Singleton, 1999).

Preparation procedures (impregnation, melting,precipitation, etc.), pre-treatment and regenerationconsiderably influence the characteristics of catalyticsystems, altering the interactions between metal andcarrier.

The possibility of regenerating the catalysts for theFischer-Tropsch reaction has been mainly examinedfor the more costly cobalt-based systems. Ironcatalysts do not have to be regenerated as a result oftheir low cost. The types of regeneration processesvary considerably in relation to the reactor solutionadopted. They range from in situ processes in the caseof re-circulating bed reactors, by enrichment inhydrogen of the gas fed, to external regenerations forfixed bed reactors, which consist in oxidation andreduction cycles.

Phenomena which lead to deactivation aregenerally different for catalysts based on cobalt oriron. For cobalt catalysts, chemical-type phenomena,including the accumulation of high molecular weighthydrocarbons which are difficult to remove, seem tobe the main reasons for their deactivation. For iron,physical deterioration phenomena are more importantthan a decrease in the catalytic activity; they causefragility with a consequent breakage of the catalyst.

The deterioration of the catalytic performanceslinked to chemical phenomena is generally due to adifferent dispersion of the metal on the surface, withrespect to the original situation, caused by the sinteringof the active phase to larger-sized aggregates. Otherreasons, which cause an alteration in the surface of thecatalyst, relate to the transformation of the metal sitesinto catalytically non-active species (metal oxides) orthe deposition of coke. The catalysts may also bepoisoned by the presence in the feedstock ofsulphurated compounds (H2S, organic sulphides),hydrocyanic acid and ammonia, the latter coming fromthe production of synthesis gas by means of processeswith air or enriched air (therefore in the presence ofN2), or by the decomposition of carbonyls of Ni and Fe(for Co catalysts) generated in the production sectionof synthesis gas or by the reaction of CO with the steelwalls of the reactor and with the internal linings. The



addition of suitable promoters can facilitate theregeneration of the catalyst by improving thereducibility characteristics necessary for removing thedeposits of carbonaceous substances on the surface ofthe metal.

Fischer-Tropsch synthesis technologiesThe Fischer-Tropsch process takes place when

synthesis gas is fed to a reactor containing the catalyst.The characteristics of the reaction products partlydepend on the type of catalyst used and partly on thereaction system. On the basis of the reactor used, thecontact mode between the catalyst and synthesis gascan vary considerably.

The selection of the reactor adopted in a certainprocess influences various characteristics such as:a) the thermal efficiency; b) heat removal;c) selectivity; d ) operating costs.

The reactors used in the Fischer-Tropsch synthesisare of the fixed bed, circulating fluid bed, fixed fluidbed and bubble column type with a mixed catalyst andthey must be capable of guaranteeing the disposal ofthe heat produced by the reaction. Temperature controlrepresents an extremely critical factor as a heatvariation inevitably has a considerable effect on thequality of the product. In fixed bed reactors, thecatalyst is charged into the reactor in the form of smalldimensioned cylinders. The cylinders must have asmall diameter in order to optimize the thermalexchange and avoid phenomena caused by rapid anduncontrollable temperature increases. Heat transfer,which is one of the critical factors in fixed bedtechnology, mainly takes place by the production ofsteam in tube bundle exchangers (Fig. 9 A). In fixed

bed reactors, the catalyst cannot be easily removed andmust therefore be stable for long periods of time.

Homogeneous transported bed reactors (Fig. 9 B)have the advantage of an optimum mass and heattransfer, which allows a uniform temperature to beobtained on the surface of the catalyst togetherwith high efficiency in the catalytic system. The critical factors, on the contrary, are theefficiency of the gas-liquid-solid mixing and the system for the separation of the catalystfrom the liquid product. Fluid bed reactors (Fig. 9 C)allow an improved heat removal and consequentlyhigher operating temperatures, which, however,can create problems by forming a carbonaceousresidue. The mass and heat transfer can beoptimally controlled using small dimensionedcatalyst particles. The recovery of the catalyst and



A B C

outletwax

outletgas

water

steam

steam

inletgas

outletliquid

inletliquid

inlet gaswater

outletgas

steam

synthesisgas

boilerfeed

water

products

catalystand tubebundles

Fig. 9. Types of Fischer-Tropsch synthesis reactors: A, fixed bed with tube bundle exchanger; B, homogeneous transported bed; C, fluid bed.

Table 3. Distribution of Fischer-Tropsch products(% C by weight)

Products LTFT HTFT

CH4 4 7

Olefins C2-C4 4 24

Paraffins C2-C4 4 6

C5-C11 cut 18 36

Medium distillates C12-C18 19 12

C18� wax and heavy oils 48 9

Oxygenates soluble in H2O 3 6

A B C

its regeneration do not create problems for therunning of the plant.

Sasol Fischer-Tropsch technologiesSasol processes for the Fischer-Tropsch synthesis

section use two different solutions: high temperaturesynthesis (HTFT, High Temperature Fischer-Tropsch)and low temperature synthesis (LTFT, LowTemperature Fischer-Tropsch). The main differencebetween the two processes is linked to the type ofproducts obtained with the two different thermalcontents; the production of a higher quantity of olefinsis associated with high temperature processes withrespect to low temperature processes, as can beobserved from Table 3 (Jager, 1998).

High temperature processes (330-350°C) areSynthol, which uses Circulating Fluid Bed Reactors(CFBR) and the more recent advanced Synthol (SAS,Sasol Advanced Synthol), with a fixed fluid bedreactor in which the product, which under the processconditions is in steam phase, and the non-convertedgas leave the reactor through internal cyclones. Withrespect to CFBR reactors, SAS reactors have theadvantage of an improved economy mainly due to theelimination of the recirculation of the catalyst and alower consumption.

There are two types of low temperature processes(180-250°C): the first uses multitubular fixed bed

reactors of the Arge type (TFBR, Tubular Fluid BedReactor), the second more recent process (SSPD,Sasol Slurry Phase Distillate; Fig. 10), usesrecirculated slurry reactors (SSBR, Sasol Slurry BedReactor). Sasol developed this technology with theaim of limiting problems linked to the running ofmultitubular fixed bed reactors; in SSBR reactors, thesynthesis gas crosses the catalytic bed consisting of adispersion of catalyst in wax, at a temperature of240°C and a pressure of 20 bar.

Table 4 indicates the average hydrocarbondistribution values for various Sasol processes(Stormont, 1960). It can be seen how, at relatively lowtemperatures, the production of heavy hydrocarbons isgreater, whereas at high temperatures the content ofolefins and low molecular weight distillates, present inthe end product, increases.

The catalysts used by Sasol are based on ironand are prepared by melting or, in more moderncatalysts, they are obtained by precipitation. Thelatter have almost completely substituted catalystsbased on molten iron and are used in all the reactorsolutions adopted by Sasol. Catalysts obtained byprecipitation can in fact be subsequently extrudedinto a suitable size for applications in fixed bedreactors or formed by atomization, into finespheroidal particles for slurry bed applications.Iron catalysts are generally promoted with Cu and



airseparation

steam methanereforming

airATR

gas

naphtha

kerosene

diesel

SSPD

naturalgas sulphur

removal

syngasconditioning

waxconversion

waxconversion

Fig. 10. Sasol SSPD process scheme, which makes use of SSPD Sasol technology, Haldor Topsøe’s Syngas technology, Fischer-Tropsch slurry-phase technology and Chevron’s conversion technology.

K and re-dispersed in a silica matrix (about 20%by weight of SiO2), in order to increase the surfacearea and the mechanical resistance.

The iron catalysts used in HTFT processes, incirculating fluid bed reactors, are subject to frictiongenerated by the fluidization of the catalytic bed andmust therefore have good mechanical resistance.

The active phase consists of iron carbide andmetallic iron produced in the reducing reactionenvironment. Under the operating conditions, thecatalyst particles are subject to the deposition ofcarbon and also to the formation of iron hydrate andthis modifies their density, influencing thefluidization properties of the bed. The carbondeposition can be controlled with the addition ofsuitable promoters which increase the life of thecatalyst.

Iron catalysts are subject to sulphur poisoning.When this affects the particle surface only, the catalyst

tends to be regenerated due to the effect of carbondeposition beneath the surface. This phenomenoncauses the detachment of outer surface, and thereforepoisoned, layers of the catalyst; when the poisoningalso extends to the internal part, the catalyst ispermanently deactivated.

Shell Fischer-Tropsch technology The Fischer-Tropsch synthesis section of the

SMDS (Shell Middle Distillate Synthesis) processuses a multi-tubular fixed bed reactor cooled withwater. The reactor operates at a temperature of 230°Cand a pressure of 28 bar, with a passage conversion of80%, in order to reduce the formation of alcohols andmaximize the formation of heavy paraffins (Fig. 11).

The catalyst consists of He balls having a diameterof about 2 mm based on silica, on which the activephase, consisting of cobalt (15-20% by weight) andoxide promoters such as TiO2 and ZrO2, is deposited.



airseparation

steam methanereforming

air

gas

naphtha

kerosene

dieselHPS

SGP

naturalgas sulphur

removal

syngasconditioning HPC

Fig. 11. Shell SMDSprocess scheme. All proprietary Shellprocesses: gas phasesyngas production (SGP, Shell GasificationProcess), syngasconversion in fixed-bedreactor (HPS, HeavyParaffin Synthesis), wax conversion (HPC, Heavy ParaffinConversion).

Table 4. Hydrocarbon distribution in Sasol LTFT and HTFT processes (% C by weight)

Products (%)LTFT-TFBR LTFT-SSBR HTFT- SAS

C5-C12 C12-C18 C5-C12 C13-C18 C5-C10 C11-C14

Paraffins 53 65 29 44 13 15

Olefins 40 28 64 50 70 60

Aromatics 0 0 0 0 5 15

Oxygenates 7 7 7 6 12 10

The synthesis techniques typically consist ofimpregnations of aqueous solutions of metal precursorsor molten cobalt salts. Shell recently disclosed thepossibility of enhancing the efficiency of the catalystby depositing the active phase only on a surface layerof the balls forming the catalyst carrier, in order toreduce the diffusive limitations (Senden et al., 1998).

ExxonMobil Fischer-Tropsch technologyThe ExxonMobil AGC-21 (Advanced Gas

Conversion for 21st Century) process is made up ofthree steps using a synthesis gas generation sectionwith a fluid bed reactor, a Fischer-Tropsch reactionsection with a homogeneous fluid bed reactor and aconversion section of the wax with a fixed bed reactor(Fig. 12). All three sections have been autonomouslydeveloped by ExxonMobil.

In its main formulation, the catalyst for theFischer-Tropsch section is based on cobalt (10-15% byweight) supported on TiO2 and promoted withruthenium or rhenium. Its preparation, according towhat is specified in the patents, is effected byimpregnation of the carrier with cobalt precursors andthe subsequent addition of the promoter. The catalystparticles are spherical and have an average diameter ofapproximately 50 mm.

Syntroleum Fischer-Tropsch technologyThe Syntroleum Corporation is a company,

founded in 1984, whose main activity is thecommercialization of a technology for the conversionof natural gas into liquid products by means of theFischer-Tropsch reaction. The main characteristic ofthe Syntroleum process is the use of AutoThermalReforming (ATR) fed with air rather than oxygen forthe production of synthesis gas. This solution has theadvantage of avoiding the cryogenic fractionation ofair, thus lowering the investment costs of the syngasgeneration section, but has the disadvantage ofproducing a synthesis gas containing nitrogen(45-50% by volume). The presence of nitrogen makesit necessary to run the Fischer-Tropsch synthesisreactor at a low operating pressure (5-7 bar) in order toavoid costly compression cycles. At low pressure, theconversion kinetics of synthesis gas are slower andextremely active catalysts are therefore required tosupport the process. Furthermore, with the sameproductivity, the reaction volumes and consequentlythe dimensions of the reactors are greater with respectto higher pressure conditions.

The catalyst described in the Syntroleum patentsconsists of cobalt with potassium as promoter (0.1-5%by weight) supported on silica, alumina or both. It is



airseparation

recyclehandling

air

gas

naphtha

diesel

HCS

HCS

FBSG

naturalgas sulphur

removal

syngasconditioning

hydrogenseparation

HI

HI

Fig. 12. ExxonMobil AGC-21 process scheme. All proprietary Exxon Processes: syngas production in fluid-bed reactor (FBSG, Fluid Bed Syngas Generation), FT synthesis in slurry bed reactor (HCS, HydroCarbon Synthesis),wax hydroisomerization (HI).

used in tubular fixed bed reactors, generally adoptingtwo reactors in series, interrupted by a condensationsection for water and higher hydrocarbons.

New approaches to the Fischer-Tropsch process:offshore plants

Between 1970 and 1990, various companiesstudied the possibility of converting the natural gasproduced in offshore plants to methanol, usingequipment assembled on ships or floating platforms.Nowadays, as about 25% of world natural gas reservesare localized in offshore reservoirs, there is someinterest in evaluating the possibility of integrating gasproduction plants on barges or naval units (FPSO,Floating, Production, Storage and Offloading), with aplant for the conversion of gas to liquid products bymeans of the Fischer-Tropsch reaction.

The potential offered by the Fischer-Tropschoffshore option concerns the possibility of exploitingrelatively small cut underwater remote gas fields(0.3-1�1012 m3) for which there are currently noinfrastructures which allow their production andtransportation. For fields of these dimensions it wouldnot be worth exploiting reserves by the production ofliquefied natural gas.

Another application which makes this methodinteresting is the exploitation of associated gas in deepwater oil fields as an alternative to gas re-injection.

Current studies, which are in quite an advancedphase, are oriented towards the development ofcompact, modular plant solutions, which are capableof tolerating undulating movement. The use ofcompact synthesis gas production technologies, basedon reforming with methane or autothermal reforming,is favoured in particular.

As far as the Fischer-Tropsch synthesis unit isconcerned, a slurry bed reactor solution is preferred asit is lighter than a multi-tubular fixed bed reactionunit. Plant capacities currently being studied are in theorder of 12,000-20,000 bpd (Apanel, 2003).

Two small licensee companies of technologies forFischer-Tropsch conversion, Syntroleum and Rentech,are amongst the most active in this field of studies. Inparticular, the former is carrying out studies onoffshore units both on barges and of the FPSO type onbehalf of the United States Department of Defence forthe production of fuel for aeronautical use in themilitary field.

Conversion of wax fromthe Fischer-Tropsch reaction

An important consequence of the hydrocarbonchain growth mechanism operating in the Fischer-Tropsch reaction is the theoretical impossibility of

producing a mixture of paraffins with a narrow chainlength range.

Regardless of the type of catalyst and operatingconditions, the Fischer-Tropsch reaction gives rise to avery varied series of products, ranging from methaneto wax consisting of high molecular weight linearparaffins. An appropriate choice of catalyst andoperating conditions allows the type of product to bevaried (paraffin/olefin/oxygenated products) withinthe restrictions imposed by the reaction mechanism.One of these implies that, in order to avoid theformation of light paraffins, it is necessary to directprocess selectivity towards the formation of extremelyheavy paraffins, as shown by the distribution indicatedin Fig. 5. It is therefore necessary for there to be areaction section, downstream of the Fischer-Tropschreaction, which converts the heavier products intomedium distillates. As the main product of thetechnology is fuel for diesel engines, it is alsonecessary to transform part of the linear paraffins intobranched paraffins, to ensure the low temperatureperformance specifications required for this fuel.

This double objective is obtained by means of theconversion process in the presence of hydrogen(hydrocracking) of Fischer-Tropsch synthesisproducts.

Historical outlines of the hydrocracking processHydrocracking is one of the oldest hydrocarbon

conversion processes. Its first application wasdeveloped by IG Farbenindustrie in Germany, in 1927,to convert lignite to gasoline. Up until the SecondWorld War, various processes were developed forproducing both liquid fuels from coal and distillatesfrom heavy charges. These processes required highhydrogen pressures (200-300 bar) and hightemperatures (�375°C). The first catalysts used werebased on tungsten sulphide; bifunctional catalysts weresubsequently used, consisting of the couples Ni/Mo,Co/Mo supported on fluorinated montmorillonite andamorphous silica-alumina (Satterfield, 1991).

After the Second World War, the hydrocrackingprocess became less important due to the wideavailability of light crude oils coming from the MiddleEast.

In the early 1960s, the availability of aneconomical hydrogen source coming from catalyticreforming and the growing demand for aviation fuelsand gasoline with high octane characteristics, led tothe development of numerous hydrocracking processesof oil fractions. Various processes of this type weredeveloped in this period by oil companies andproducer and technology licensee companies(Stormont, 1959; Sterba and Watkins, 1960; Jager,1998). In the 1970s, there was a rapid growth of these



processes, especially in the United States,accompanied by continuous improvements in both thecatalysts used and also the process schemes. Thegrowth continued in the 1980s and 1990s, but at aslower rate and the major development areas were theMiddle East, Asia and the Pacific Ocean.Hydrotreatment processes currently form 40-50% ofthe world’s refinery capacity.

Reaction chemistryHydrocracking catalysts are of a bifunctional

nature i.e. they are characterized by the presence ofacid sites which carry out an isomerization/crackingfunction and metal sites which have ahydro/dehydrogenating function.

Typical acid carriers are: amorphous oxides ortheir mixtures (F/Al2O3, SiO2/Al2O3, ZrO2/SO4

2�),zeolites (crystalline alumino silicates, porous),mixtures of zeolites and amorphous oxides.

Metals which have a hydro/dehydrogenatingfunction can be noble metals (Pt, Pd) or sulphides ofnon-noble metals of group VIA (Mo, W) and groupVIIIA (Co, Ni). Catalysts containing metal sulphidescurrently form an overwhelming majority ofcommercial hydrocracking catalysts, as they areinsensitive to the presence of the sulphuratedcompounds normally present in the refineryfeedstocks (Giusnet et al., 1987).

The mechanism of hydrocracking reactions onbifunctional catalysts has been the object of numerousstudies and still represents an active field of research.Most of the work has been effected using modelcompounds, fundamentally n-paraffins, and to a lesserextent naphthenes, alkyl-aromatic and polychromaticcompounds (Weitkamp et al., 1984).

As the Fischer-Tropsch reaction almost exclusivelyproduces n-paraffins, in this context studies relating tothis group of compounds alone will be examined.

It is generally accepted that the reaction in questiontakes place through a mechanism of the carbo-cationictype which envisages a hydro/dehydrogenation stepand a skeleton isomerization and hydrocarbon chainbreakage step.

The hydrocracking of n-paraffins passes throughthe following phases: a) adsorption of n-paraffins onthe metal site; b) dehydrogenation with the formationof the n-olefin; c) deadsorption of the n-olefin anddiffusion towards acid sites; d ) isomerization and/orcracking of the olefin on the acid sites using a carbo-cationic intermediate; e) deadsorption of the olefins bythe acid sites and diffusion towards the metal sites;f ) hydrogenation of the olefins; g) deadsorption of theiso- and normal-paraffins. The elemental reactionscorresponding to the reaction scheme described above,are indicated in Fig. 13.

Unlike what is specified above, this generalscheme also considers the presence ofhydrogenolysis reactions on the metal sites.According to this reaction mechanism, theformation of iso-paraffins with a number of carbonatoms equal to that of the converted n-paraffin takesplace by the re-arrangement of the secondarycarbocation to tertiary, via a cyclic intermediate,and the subsequent formation of the iso-olefin. Thecracking reaction, on the other hand, is effected bythe extraction of hydrogen and breakage of theparaffinic chain with the formation of one paraffinand one olefin lighter than the starting hydrocarbon(b-scission). In order to explain the formation of thedifferent types of branching of iso-paraffins intocracking products, various types of b-scissions havebeen identified and the mechanisms proposedsuggest that n-paraffins can undergo severalisomerizations before having a configuration whichis favourable for b-scission (Weitkamp, 1982).Some authors have recently proposed that bothcracking and isomerization reactions take placethrough a common intermediate like protonateddialkylcyclopropane (Tiong Sie, 1992, 1993). Theselectivity towards isomerization and thedistribution of cracking products greatly depend onthe type of catalyst used. A catalyst with a weakhydrogenating function (for example Ni or Wsulphides) on an acid carrier such as Al2O3/F orSiO2/Al2O3 shows low selectivity for isomerization.Much better results are obtained when a metal with



n-alkanes cracking products

n-alkenes

rearrangement

b-scission

b-scission

cracking productsn-alkyl secondary carbocations

i-alkenes cracking productsi-alkyl tertiary carbocations

hydrogenolysis

i-alkanes cracking productshydrogenolysis

Fig. 13. Reactionscheme for thehydrocracking/hydroisomerization of n-paraffins on a bifunctionalcatalyst.

a high hydrogenating function is used (Pt, Pd) tobalance the acidity of the carrier.

The distribution of the products obtained fromhydrocracking n-hexadecane with various catalysts,differing in their hydrogenating component andcarrier, is shown in Fig. 14 (Weitkamp and Ernst,1990).

A catalyst with a higher hydrogenating/acidityfunction ratio (Pt/CaY) gives rise to a distributionin which the C4 to Cn�4 fragments are produced inalmost equimolar quantities; there is a limitedpresence of C3 and Cn�3 fragments and there is noformation of C1, C2, Cn�1 and Cn�2 fragments; theproduction of low molecular weight fragments islower compared to higher molecular weightproducts. A situation of this kind is defined asideal hydrocracking and produces high yields toliquids.

A catalyst characterized by a lowhydrogenating/acidity function ratio(Co-Mo-S/SiO2-Al2O3), on the other hand, shows adistribution which is shifted towards light products dueto the presence of secondary cracking.

The reactivity of the single n-paraffins (Table 5)rises with an increase in the chain length on bothcatalysts with an amorphous carrier and zeolites(Weitkamp and Ernst, 1990).

Fig. 15, on the other hand, shows that selectivitytowards isomerization decreases with an increase inchain length.

In order to achieve the objective of maximizing theyields to medium distillates and obtain high-qualityproducts, the cracking process must have the followingthree characteristics:• The chain length of the cracking products must

prevalently correspond to that of the desired range



mol

es p

er 1

00 m

oles

C16

cra

cked

0

20

40

60

80

100

120

140

carbon number of cracked products

catalytic cracking: SiO2-Al2O3-ZrO2 T�500°C; conv.�54%

hydrocracking: Co-Mo-S/SiO2-Al2O3 °C; conv.�50%

hydrocracking: Pt/Ca Y T�230°C; conv.�55%

2 4 6 8 10 12 14

Fig. 14. Productdistribution of catalyticcracking andhydrocracking of n-C16at 50%.

Fig. 15. Selectivity to isoparaffins for the conversion (from the bottom upwards) of n-C36, n-C28 and n-C16 onPt/MSA catalyst at 380°C.

sele

ctiv

ity

to is

o-pa

raff

in

0

0.2

0.4

0.6

0.8

1.0

conversion0 0.2 0.4 0.6 0.8 1.0

Table 5. Reactivity of n-paraffins with variationof the chain length

Reagent1ST order constant for the

formation of cracking products(arbitrary units)

n-C10 1.0

n-C11 1.8

n-C12 –

n-C13 –

n-C14 10

n-C15 22

n-C16 37

n-C17 87

i.e. the distribution of the hydrocracking productsmust be of the ‘ideal’ type.

• Chains with a greater length than that of thedesired range should have much higher selectivity.

• The catalyst must have high isomerizing capacityin order to obtain fuels with good coldperformance characteristics.Generally speaking, the conditions listed above are

more or less satisfied when an optimum balance isobtained, in the catalyst, between the metallicfunctionality and the acidic nature of the carrier.

Conversion technologies Shell was one of the first industries to study and

develop a hydrocracking technology oriented towardsthe conversion of wax produced by the Fischer-Tropsch reaction (Hoek et al., 1984). The Shellprocess scheme, carried out for the first time atBintulu, in Malaysia, envisages the preheating of thestream of paraffins with more than six carbon atoms,coming from the Fischer-Tropsch reactor, to thereaction temperature, mixing it with hydrogen andsubsequently sending it to the hydrocracking reactor.The outgoing products are sent to a separator, in whichthe non-reacted hydrogen is recovered and the lowmolecular weight products (�C4) are separated.Finally, the remaining liquid fraction is fractionated inthe distillation section and the heavy paraffins whichhave not been converted, are recycled to the crackingreactor.

The process adopts a classical fixed bed crackingreactor operating at a temperature of 300-350°C anda pressure of 50-130 bar. Hydrogen consumption isapproximately 300 scf/bbl. This value isconsiderably lower than the values obtained for thehydrocracking of refinery oil fractions (500-3,000scf/bbl). During the reaction, in addition to thehydro-isomerization and hydrocracking of thefeedstock, hydrogenation of the olefins and removalof oxygenated compounds, mainly consisting ofprimary alcohols, also take place.

2.6.3 Products

The technology for the conversion of gas into liquidproducts by means of the Fischer-Tropsch reaction isgenerally oriented towards the conversion of largequantities of gas, in order to benefit from theeconomical advantages deriving from the plant scale.In this context, the main products obtained must havewide consumption, such as, for example, aviation fuelsand diesel engines.

Together with these basic products, it is possiblehowever to direct the hydrocracking section towardsthe production of special high value-added products,destined for applicative fields different from fuels. Inprinciple, these products are those which give themaximum economic advantage and the limit to thequantity produced is determined by the receptivity ofthe destination market. A description is providedbelow of the upgrading of the products which can beobtained.

Basic productsThe basic products consist of the fraction of

products which are gaseous at room temperature,similar to LPG, and the liquid fraction which can beclassified on the basis of the boiling range in naphtha,kerosene and fuels for diesel engines. An accuraterunning of the hydrocracking section allows theproduction of the diesel fraction or kerosene fractionto be maximized, whose typical overall yields areindicated in Table 6.

LPGLight gaseous fractions are an inevitable but

undesirable product for economic reasons. Thesefractions can either be sent to the local market, whenexisting, or recycled to the synthesis gas generationsection, or they can be sent to a burner for producingthe energy necessary for the process.

NaphthaThe cracking process in the presence of naphtha

vapour represents the main technology adopted,especially in Europe, for the production of lightolefins. The feed used is normally so-called virginnaphtha, a light oil fraction (boiling point 38-190°C)with a variable composition depending on thestarting crude oil. The characteristics of the C5-C9

fraction coming from primary distillation or thehydrocracking of wax produced in theFischer-Tropsch reaction make it similar to refineryvirgin naphtha. Compared to the latter, it has bettercharacteristics due to the fact that, as it exclusivelyconsists of n-paraffins, it can give a higher yield toethylene and propylene with respect to the refinery



Table 6. Composition of the liquid productsobtainable by gas conversion, through Fischer-Tropsch

synthesis and hydrocracking

Liquid productsMaximum diesel

(% vol.)Maximum kerosene

(% vol.)

Naphtha 15 25

Kerosene 25 50

Diesel 60 25

Total 100 100

feedstock which also contains i-paraffins, cyclo-paraffins and aromatic compounds. A recent studycarried out by Sasol, Chevron-Texaco and KellogBrown & Root, confirmed the high quality of thenaphtha produced via Fischer-Tropsch whether inhydrogenated form or not.

As far as the boiling point is concerned, thisfraction could also be used as gasoline. However, dueto the linear paraffinic nature of the product, theoctane number of the resulting gasoline is extremelylow. From an economic point of view, it is worth tryingto minimize the production of this distillation cut.

KeroseneThe primary distillation fraction mainly consists of

n-paraffins and consequently has an inadequate pourpoint for use as aircraft fuel. The hydrocrackingprocess of wax, on the other hand, is optimized to alsoprovide the desired branching degree required forproducts within specification. The smoke point of thisfraction is very high and this property is extremelyinteresting as specifications on this value require asmoke point of at least 25 mm. A critical aspect of thisfraction can be its freezing point.

The specification for a fuel for class Jet A-1aircraft is a freezing point lower than �47°C. Theproduct obtained by means of the Fischer-Tropschsynthesis has a freezing point close to specificationand this aspect could be critical for mixtures ofconventional refinery products with products derivingfrom the Fischer-Tropsch synthesis. The typical

properties of a kerosene obtained from theFischer-Tropsch reaction are indicated in Table 7.

Diesel fuelDiesel fuel obtained by Fischer-Tropsch synthesis

is virtually free of sulphur and aromatic compounds;furthermore, as it is considerably paraffinic, it has anextremely high cetane number. The cetane numbercharacterizes the combustion properties of a dieselfuel. A low cetane number can be responsible forincomplete combustion, especially at low temperaturesi.e. in the ignition phase of the engine. A high cetanenumber, on the other hand, improves combustion andtends to reduce the formation of NOx, CO and powdersin the emissions. The absence of sulphur and aromaticproducts also improves the efficiency of thepost-treatment systems of discharge gases. Thanks toits properties relating to the content of polyaromaticproducts, density and cetane number, Fischer-Tropschfuel can be useful for refiners to improve low-qualityrefinery streams through mixing in appropriateproportions. It is not possible to use a Fischer-Tropschcut as such as diesel fuel, mainly due to its lowdensity. The characteristic properties of a diesel fuelproduced by the technology based on theFischer-Tropsch reaction are indicated in Table 8.

Special products

C10-C16 fraction. Paraffins for linear alkyl-benzenes Linear Alkyl Benzenes (LAB) are the main

feedstock for the production of the correspondingsulphonates, which are widely used as surface-activeagents for detergents. They are commercially producedfrom the paraffins obtained by fractionation of thekerosene cut by the dehydrogenation and alkylation ofbenzene, chlorination and alkylation of benzene,chlorination followed by dihydrochlorination andsubsequently alkylation of benzene.

The C10-C13 cut coming from a Fischer-Tropschunit is comparable to the paraffins obtained from thekerosene fraction. It is also rich in n-paraffins and freeof sulphur and aromatic hydrocarbons and cantherefore be used as feedstock for the production ofLAB.

In 2002, the world demand for these products wasslightly less than 1 million tons.

C17-C22 fraction. Drilling fluids Drilling fluids (SBM, Synthetic Based Mud) are

mainly used for carrying the drill cuttings generatedduring drilling to the surface and for lubricating andcooling the whole drilling rod system.

SBMs represents a new category of fluids recentlydeveloped to overcome critical aspects, such as



Table 7. Typical properties of a kerosene producedby means of the Fischer-Tropsch process

Density 750 kg/m3

Smoke point �50 mm

Sulphur �1 ppm

Aromatics �1% in vol.

Table 8. Typical properties of a diesel fuel producedby means of the Fischer-Tropsch process

Cetane number �70

Total aromatics (% vol.) �3

Distillation temperatureT90 max (°C)

320

Sulphur (ppm) �5

Density (kg/m3) 780

environmental impact or limited performances,revealed by the use of conventional oil- or water-basedfluids. The main objective of the introduction of SBMis to make use of high performance fluids which allowthe disposal of drill cuttings without the pretreatmentnormally required in the use of oil-based fluids. TheC17-C22 cut deriving from a Fischer-Tropsch product isa useful component, after suitable treatment and/ormixing with additives, for reaching the specificationsrequired in the formulation of drilling fluids of theSBM type. A set of regulations is being defined by theAmerican Environmental Protection Agency (EPA) forthe use of SBM of various origins, comprising thosecoming from gas conversions by means of theFischer-Tropsch reaction.

C18-C22 fraction. Production of refined paraffinicwaxes

Refined paraffinic waxes are a mixture of solidsaturated hydrocarbons corresponding to the C18-C22

fraction in a mixture with heavier cuts. They are usedin the production of candles, packaging materials,tyres, seals and powder paints.

The linear structure which characterizes theFischer-Tropsch synthesis product suggests that it canform an ideal component for the production of paraffinicwaxes. The world market for synthetic waxes derivedfrom oil was estimated at 3.5 million t/a in 2002.

C18-C22 fraction. White oilsWhite oils are among the purest products derived

from petroleum; they are a mixture of aliphaticsaturated hydrocarbons which are colourless,odourless, tasteless and chemically stable within awide temperature range. They are normally producedby treatment of an oily base with acids or withhydrogen at a high temperature and pressure. Theirmain uses are in the pharmaceutical, food, cosmeticand personal hygiene industries.

The C18-C22 fraction produced by means of theFischer-Tropsch synthesis forms an excellent startingproduct for the production of these oils, even if it mustbe subjected to treatments to obtain the characteristicsrequired.

C23+ fraction. Lubricating basesLubricating bases, mixed with suitable additives to

obtain the desired properties, give rise to theproduction of finished lubricants; they can be dividedinto conventional, non-conventional and synthetic.

Non-conventional bases are produced by theisomerization of synthetic paraffinic waxes, thetreatment of residues from hydrocracking and theoligomerization of a-olefins for the production ofpolyalphaolefins (PAO).

A key property of lubricating bases is the viscosityindex which is a measurement of the variation inviscosity in relation to the temperature; the higher thisindex, the greater the capacity of the oil to maintainviscosity at a high temperature. The most suitablecomponents for this purpose are isoparaffins whichhave a high viscosity index, a low pour point, plusgood thermal and oxidation stability. The lubricatingbases from wax obtained by means of Fischer-Tropschsynthesis have higher properties than the bases ofgroup III API (which consist of bases obtained fromadvanced hydrotreatment processes) and have a lowsulphur content, a high content of saturatedhydrocarbons and a high viscosity index (�120).Fischer-Tropsch bases can be compared with PAOwhich are part of group IV API. The bases, obtainedby the isomerization of the C23� fraction, produced bymeans of Fischer-Tropsch synthesis, have an excellentviscosity index and, in absolute terms, higher thanmany other non-conventional bases; these bases alsohave a high boiling point for each viscosity cut andthis property gives them a low volatility (a low Noackvalue, which is the volatility measurement of an oil)and a high flash point.

The main disadvantage of these bases is the pourpoint which can be too high in the heavier cuts. Thisdrawback can be easily adjusted by the increment ofmethacrylate type additives. On the whole,Fischer-Tropsch synthesis bases are preferable to PAO,which have very high production costs, and which,unlike the former, require the addition of esters toallow them to be used in engine oils.

The world demand for synthetic lubricating baseswas about 1 million tons in 2002.

References

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Giuseppe BellussiRoberto Zennaro

EniTecnologieSan Donato Milanese, Milano, Italy



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